Glass Interleaver Paper Produced With Coarse Fibers

ABSTRACT

A paper for use as a slip sheet interleaver for the protection of glass surfaces in which the paper comprises a sheet with a basis weight of 15-60 lb/3,000 ft2, has a bulk density less than 0.75 g/cm3, and has an average fiber coarseness greater than 0.18 mg/m. In comparison to conventional interleaver sheets, this paper is produced from a refined pulp having high coarseness.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 62/519,698 entitled “Glass Interleaver Paper Produced with Coarse Fibers” filed on Jun. 14, 2017. That application is hereby incorporated by reference for all purposes as if set forth in its entirety herein.

STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

This disclosure relates to interleaver sheets for the separating sheets of glass in order to protect the glass from breakage and surface damage.

Glass sheet products are commonly packaged in packs or stacks with an interleaving sheet separating the sheets of glass. The purpose of the interleaver sheet is to protect the glass from breakage, surface damage such as scratches or stain spots, and to allow for ease of handling and separation of the glass during unpacking.

For technical glass products in particular, such as the glass used in video displays, the interleaver ideally protects the glass from developing even tiny surface defects caused by contaminants in the interleaver or from contaminants created during the glass making, packing, transporting, and use processes. Such interleaver paper should be free of contaminants such as hard particles, which can cause a scratch or abrasion spot in the glass surface, free of contaminants that will transfer to the glass and are not easily removed, and cause loose particles or a “stain” on the glass. As display technology has advanced, the protection of the glass for the display has become increasingly difficult because as the resolution of such displays increases (e.g., from 720 to 1080 to 4K to beyond in future generations of displays), the size constituting a defect in the glass decreases in proportion to the resolution. Therefore, as the quality demands on glass continue to increase, especially on LCD/OLED glass, higher performing interleaver paper is required.

SUMMARY OF THE INVENTION

In view of the above-described state of the art, conventionally, to inhibit the presence or impact of contaminants, interleaver papers have been designed with northern softwood ‘soft’ fibers to have low bulk, to have very fine smooth surface texture, and to minimize particle generation and migratable materials.

This disclosure presents a different and counter-intuitive approach to production and composition of interleaver paper which has been found to have improved protection performance over the conventional paradigm that interleaver paper should have exceptionally smooth contact surfaces. Instead of attempting to maximize the bulkiness and construct the interleaver paper with very fine fibers and an extremely smooth surface texture, the interleaver paper disclosed herein employs coarse, long fibers with optimized refining to produce a sheet with uniform thickness, with reduced density, and with a higher roughness surface suitable as a high performance glass interleaver. In many forms, the pulp containing coarse fibers can be refined to provide a high percentage of short length fiber fractions. Production of paper with these fibers has resulted in a uniform thickness sheet with even higher bulk, higher but more uniform surface micro-roughness, high surface strength, and good physical properties that have been discovered to be well-suited for a high performing glass interleaver.

The newly-disclosed interleaver paper is especially suitable for LCD glass protection during shipping and use because it is posited that, among other things, the greater bulkiness cushions the glass layers and minimizes scratching, the roughness (smoothness) minimizes scratching, and the internal bonds prevent fiber transfer and particle generation. Based on the current state of the art, it is counter-intuitive to prevent scratching by making the sheet with coarse fibers and high micro roughness. In practice, it has been found that the uniform (but comparably higher) surface micro-roughness provides a multitude of support peaks separated by valleys. Thus, when contacting the glass, the micro-roughness is akin to laying on a bed of one thousand nails rather than on a surface with only two or three contact points. Less damage to the glass is likely to occur when the load is evenly distributed over the multitude of contact points instead of only two or three contact points. Still further, the higher micro-roughness also provides valleys into which particles can fall into such that the particles are unable to touch the glass and scratch it. This is in contrast to conventional high-density interleaver papers that do not have volumes for the innocuous reception of hard contaminants.

According to one aspect, a paper is disclosed that is used as a slip sheet interleaver for the protection of glass surfaces. The paper comprises a sheet with a basis weight of 15-60 lb/3,000 ft², having a bulk density less than 0.75 g/cm³, and an average fiber coarseness greater than 0.18 mg/m.

In some forms, the sheet may comprise at least 20%, 80%, or 100% coarse fibers, for example. The coarse fibers may comprise a set of fibers selected from a group consisting of southern softwood kraft, southern hardwood kraft, mercerized fibers, Coastal Douglas Fir, Radiata Pine, and synthetic polymeric fibers. Still further in some specific forms, the sheet may comprise southern softwood and comprise 20, 80, or 100% fluff pulp.

In some forms, the sheet may comprise at least 10% coarse fibers and those coarse fibers may include one or more of southern softwood kraft, southern hardwood kraft, mercerized fibers, Coastal Douglas Fir, Radiata Pine, Fluff Pulp, and synthetic polymeric fibers.

In some forms, the sheet may comprise at least 80% coarse fibers and those coarse fibers may include one or more of southern softwood kraft, southern hardwood kraft, mercerized fibers, Coastal Douglas Fir, Radiata Pine, Fluff Pulp, and synthetic polymeric fibers.

In some forms, the sheet may have a basis weight of 25-40 lb/3000 ft², a bulk density less than 0.65 g/cm³, and an average fiber coarseness greater than 0.20 mg/m.

In some forms, the sheet may have a Scott Bond strength greater than 200 g, 250 g, or 300 g.

In some forms, the sheet may have a Parker smoothness greater than 7.5 μm on both sides or a Parker smoothness greater than 8.5 μm on both sides.

In some forms, the sheet may have a surface roughness (Sa) greater than 4.5 μm on both sides or a surface roughness (Sa) greater than 5.0 μm on both sides.

In some forms, the sheet may have a void volume greater than 1.30 g/g or 1.40 g/g.

In some forms, the sheet may have an average fiber length (LWFLA) less than 1.9 mm or an average fiber length (LWFLA) less than 2.1 mm.

In some forms, the sheet may have a short fiber and fines content greater than 10% (in which the short fibers and fines are less than 0.5 mm). The sheet may also have a long fiber content greater than 5% (in which the long fibers are greater than 3.5 mm). In some forms, the sheet may have a short fiber and fines content greater than 10% (in which the short fibers and fines are less than 0.5 mm) and simultaneously have a long fiber content greater than 5% (in which the long fibers are greater than 3.5 mm).

In some forms, the sheet may have a content of medium length fibers between 0.9 mm and 2.7 mm of less than 55%.

In some forms, the sheet may have a short fiber and fines content greater than 15% (in which the short fibers and fines are less than 0.5 mm).

In some forms, the sheet may have less than 0.5 parts per million (ppm) poly dimethyl siloxane fluid (PDMS) content.

In some forms, the sheet has less than 0.35% ash content.

In some forms, the paper may comprise a binding agent. In some forms, the binding agent may be selected from the group of adhesives including acrylic latex, styrene butadiene copolymer, butadiene acrylonitrile copolymer, polyurethane, polyvinyl acetate, polyvinyl alcohol, natural rubber or other nature-based adhesive, polyvinyl chloride, polychloroprene, epoxy, phenol, urea-formaldehyde, and thermal melt adhesive. In some forms, the binding agent may be fiber including at least one polymer selected from the group of polymers consisting of polyolefin, polyester, polyamide, polylactide, polycaprolactone, polycarbonate, polyurethane, polyvinyl acetate, polyvinyl chloride, polyvinyl alcohol, polyacrylate or polyacrylonitrile, and ionomer.

According to another aspect, a method of making the paper described above includes the use of refining equipment, refining energy level, and fiber types to produce the paper comprising fibers having a wide fiber length distribution in which the sum of short fibers and fines and long fibers is greater than 20% of the fiber mix in which the short fibers and fines are less than 0.5 mm and in which the long fibers are greater than 3.5 mm.

According to yet another aspect, a method of making the paper described above includes the step of using of a paper machine forming section to make the paper, in which the forming section includes at least one of a Fourdrinier, Inclined Wire Former, Cylinder Former, Twin Wire Former, Gap Former, Top Former, Multi-layer Former, and Tanmo.

According to still another aspect, a method of separating sheets of glass using the paper described above is disclosed comprising the step of separating two sheets of glass by positioning the paper therebetween.

It will be appreciated that while various compositions and parameters are recited above and herein separately, that all workable combinations and permutations of such compositional requirements and resulting parameters are contemplated as falling within the scope of this disclosure.

These and still other advantages of the invention will be apparent from the detailed description. What follows is merely a description of a preferred embodiment of the present invention. To assess the full scope of the invention, the claims should be looked to as the preferred embodiment is not intended to be the only embodiment within the scope of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the paper surface roughness profile of paper made from Northern bleached softwood kraft pulp (NBSK).

FIG. 2 illustrates the paper surface roughness profile of paper made from Coarse Pulp.

DETAILED DESCRIPTION OF THE INVENTION

The following terms will be used in this application:

The basis weight is a measure of the weight of a paper per area. Typical units are pounds per 3000 ft² or grams per square meter (gsm).

Bulk is the opposite of density. Sheets with low density are said to have high bulk.

Calendering is a process of exposing a sheet of paper to pressure and heat in order to further densify, smooth, and consolidate the sheet.

Caliper is thickness of a paper. Typical units are mils (thousandths of an inch) or microns.

A defoamer is a chemical used in the pulping process to reduce/minimize foam. Most commonly these contain poly dimethylsiloxane (PDMS) oil as the base for the defoaming capability.

Density is a measure of the mass per unit volume of a paper. Density can be calculated by dividing the basis weight by the thickness of a paper. Typical units of density are grams per cubic centimeter.

The felt side is the surface of paper which is opposite the side facing the forming fabric during the drainage process (with the opposite side being referred to as the wire side).

Fiber coarseness is the measure of the mass of fibers expressed in units of mass per length (mg/m). Fiber coarseness is dependent upon fiber wall thickness.

Fiber length (arithmetic mean) is the sum of all the fiber lengths divided by the number of fibers. Arithmetic mean fiber length is typically expressed in millimeters.

Fiber length (length weighted average) is a fiber length calculation which reduces the significance of shorter fibers in the distribution—which is typically not uniform. Length weighted average fiber length is typically expressed in millimeters.

Fiber wall thickness is the thickness, typically measured in microns, of the outer wall of a wood fiber.

Fines are fibers or portions of fibers less than 0.2 mm in length.

Fluff pulp is long, coarse fibers, typically softwood kraft, more specifically, slash pine. Fluff pulp is commonly used in absorbent products and airlaid application.

Glassine is a highly densified sheet of paper which is typically produced using high levels of fiber refining and supercalendering.

Lumen diameter is the width of the round center hollow portion of a wood fiber.

Mercerized pulp is a wood pulp which has been treated with a strong alkali such as caustic soda, which results in a fiber with a high degree of kinks or bending. Mercerized pulp is typically used in high bulk, highly breathable applications such as filtration papers.

Micro-roughness is a measure of the sheet surface roughness. Micro-roughness may be measured using Vertical Scanning Interferometer (VSI) technique.

MorFi fiber analyzer is an instrument which determines fiber properties of a mix of fibers by using optical methods as described by standard test method ISO 16065-2.

“NBSK” refers to Northern bleached softwood kraft pulp.

Parker smoothness is a measure of surface smoothness and is also known as Parker Print Surf (PPS), standard Tappi method T555. Units of Parker smoothness are in microns with higher values indicating less smooth surfaces.

Percent ash is amount of ash remaining after paper is placed in oven higher than 525 degrees C. according to standard method Tappi T211.

Refining is a process of applying energy to develop wood pulp fibers for papermaking. Effects of refining include cutting of fibers, bruising of fibers, collapsing of fibers, creating fines, and creating fibrils.

Refining energy is a measure of the amount of energy applied during the refining process to a stream of wood pulp, typically expressed at units of horsepower-days per ton. Most papers are produced using a refining energy of up to 10 hp-day/ton.

“SBHK” refers to Southern bleached hardwood kraft pulp.

“SBSK” refers to Southern bleached softwood kraft pulp.

Scott Bond strength is paper sheet strength measured in the direction of the thickness (z direction) of the sheet. The method of measurement involves placing two-sided tape on the surfaces of the sheet and measuring the force required to split the sheet. The method of testing is also known as Internal Bond and is standard method Tappi T569.

Silicone refers to synthetic polymers whose structure is based on repeating units of siloxane, which is a chain of alternating silicon and oxygen atoms, such as poly dimethyl siloxane fluids. Silicone backbone polymers may also be polymerized with, have side chains or other functional groups added for specific chemical or performance in use properties.

Supercalendering is an extreme method of calendaring utilizing high heat and pressure and a number of nips. Supercalendering is typically used in the manufacture of pressure sensitive release liners and glassines.

Surface roughness (Sa) is the overall surface roughness as measured by a Bruker NPFlex Vertical Scanning Interferometer according to standard method ISO 25178. Typical units of surface roughness are in microns with the higher the number, the rougher the sheet surface.

Void volume is a measure of the volume of empty space within a paper sample, as measured by saturation with a standard wetting agent. This test method is described in detail in U.S. Pat. No. 7,794,566, which is incorporated herein by reference. In pertinent part, U.S. Pat. No. 7,794,556 explains that void volume is determined by saturating a sheet with a nonpolar liquid and measuring the volume of liquid absorbed. The volume of liquid absorbed is equivalent to the void volume within the sheet structure. The void volume is expressed as grams of liquid absorbed per gram of fiber in the sheet. More specifically, for each single-ply sheet sample to be tested, eight sheets are selected and 1 inch by 1 inch squares are cut out (1 inch in the machine direction and 1 inch in the cross-machine direction). For multi-ply product samples, each ply is measured as a separate entity. Multi-ply samples should be separated into individual single plies and eight sheets from each ply position are used for testing. The dry weight of each test specimen is weighed and recorded to the nearest 0.001 gram. The specimens are placed in a dish containing POROFIL® pore wetting liquid of sufficient depth and quantity to allow the specimen to float freely following absorption of the liquid. POROFIL® liquid, having a specific gravity of 1.875 grams per cubic centimeter, is available from Quantachrome Instruments, 1900 Corporate Drive, Boynton Beach, Fla. 33426. After 10 seconds, the specimen is grasped at the very edge (1-2 millimeters in) of one corner with tweezers and is removed from the liquid. The specimen is held with that corner uppermost and excess liquid is allowed to drip for 30 seconds. The lower corner of the specimen is lightly dabbed (less than ½ second contact) on filter paper in order to remove any excess of the last partial drop. The specimen is immediately weighed within 10 seconds, and the weight is recorded to the nearest 0.001 gram. The void volume for each specimen, expressed as grams of POROFIL® per gram of fiber, is calculated as follows:

Void Volume=[(W ₂ −W ₁)/W ₁]

in which W₁ is the dry weight of the specimen in grams and W₂ is the wet weight of the specimen, in grams.

Wire side is the surface of a sheet of paper which is facing the forming fabric (the wire) during the drainage process.

With this terminology having been generally established, the improved paper composition for the interleaver sheet is now described in greater detail.

After attempts to further improve existing paper formulas for production of interleaver sheets to separate glass sheets—of which all utilized northern fibers—yielded no better performance results than existing formulations, the inventors adopted a completely different approach in which refined pulp having high coarseness fibers would be formed into interleaver sheets.

This is a highly unobvious approach. Pulps with high coarseness fibers tend to be used in absorbent products and are not typically refined, as this reduces the bulk and thus absorbency properties. For example, PCT Application Publication No. WO 01/57313 explains the difficulty in producing sheets with high levels of fluff pulp due to the lack of bonding with coarse fibers. Still further, the best fibers, according to the current state of the art, to produce a bulky, soft sheet such as a bath tissue or toweling, are Eucalyptus bleached kraft, or NBSK fibers with low coarseness. See, for example, U.S. Patent Application Publication No. 2106/0244916. Further, JP4313415B1 states that fibers less than 1.0 mm result in poor cushioning effect with any cushioning effect resulting from fiber lumens that are easily collapsed (i.e., thin wall, wide lumen).

Thus, historically, pulps containing high coarseness fibers were not deemed suitable for production of glass interleaver sheets because of their poor ability to form a uniform sheet and to form a smooth sheet surface to prevent damage of the glass and further because they were not believed to be particularly cushioning.

Laboratory work was performed to determine the usefulness of various pulps as a high performing glass interleaving sheet. In this study, four different pulps were used and refined to varying levels. Each of these conditions then was used to produce lab made paper sheets called handsheets. The test results of the handsheets can be used to show relative differences. The four pulps chosen for the lab work exemplify the range of common commercially available papermaking fibers. The pulps tested included Southern Bleached Hardwood Kraft (SBHK), Northern Bleached Softwood Kraft (NBSK), Southern Bleached Softwood Kraft (SBSK), and Fluff Pulp. Each pulp was refined using a benchtop refiner known as a PFI mill, described in Tappi Test Method T248, Laboratory beating of pulp (PFI mill method). Handsheets were made prior to refining, and after 1500, 2250, and 3000 revolutions. Properties of the handsheets were tested to determine the suitability of each pulp for the manufacture of glass interleaving paper. These properties include Density, Parker Smoothness, Scott Bond, Void Volume, and Surface Roughness. The results of the study are shown in Table 1.

TABLE 1 Properties of Handsheets Based on Pulp Type and Refining Parker Surface Scott Void Refining Pulp Density Smoothness Roughness Bond Volume Level Type (g/cm3) (um) (um) (g) (g/g) Zero SBHK 0.45 8.02 6.95 30 2.97 Refining NBSK 0.47 8.18 6.72 52 2.47 SBSK 0.44 9.78 7.85 44 2.56 Fluff 0.39 11.69 10.62 37 2.95 Pulp Low SBHK 0.50 8.17 6.19 75 2.19 Refining NBSK 0.54 8.45 5.63 130 1.73 1500 SBSK 0.52 10.28 6.99 125 1.83 rev. Fluff 0.49 11.03 7.68 117 2.00 Pulp Medium SBHK 0.53 8.39 5.43 127 1.96 Refining NBSK 0.55 8.93 5.60 200 1.50 2250 SBSK 0.52 10.41 6.93 150 1.73 rev. Fluff 0.51 11.37 7.11 135 1.83 Pulp High SBHK 0.58 8.70 5.28 191 1.65 Refining NBSK 0.56 8.81 5.41 208 1.57 3000 SBSK 0.53 10.57 6.51 197 1.55 rev. Fluff 0.52 11.46 6.89 178 1.67 Pulp The results of this study show that for the key performance measures listed above, Fluff pulp is the best overall performer of all the pulps tested. Southern Bleached Hardwood Kraft is shown to have good void volume and low density at lower refining levels, but low Scott bond and sheet strength makes the paper prone to fiber transfer to the glass or tearing when removed from the glass respectively. Northern Bleached Softwood Kraft can be used to make paper with good internal bond, but paper from NBSK is more dense, has low void volume, but is too smooth to be a high performing glass interleaver since the surface valleys are too shallow to keep particles from contacting the glass. Southern Bleached Softwood Kraft is the second best performing pulp, but is outperformed by fluff pulp in levels of Density, Parker Smoothness, Surface Roughness, and Void Volume.

In order to verify the laboratory results, the trial interleaver papers were produced from pulps including high coarseness fibers including “fluff” pulp and a mercerized pulp. Fluff pulps are typically used in absorbent products such as air-laid nonwovens and other absorbent type pads. Mercerized pulps are typically used in filtration applications due to their extreme bulkiness and ability to create high air or liquid flow through a substrate. These fibers were chosen based on superior cleanliness as well as bulking abilities. It is contemplated that other coarse fibers might also be employed in formulations including southern hardwood kraft (from southeastern United States), Radiata pine (from South America), and Chinese red pine (Southeast Asia and China). It is noted that coarse fibers such as in fluff pulp, specifically from pine species growing in the southeast United States, including slash, loblolly, longleaf pine, or other coarse fibered softwood species, have not been known to be used in pulp formulations for glass interleaver sheets and the current state of the art suggest that low coarseness fibers are better in such applications for cushioning/scratch prevention because they result in a smoother surface.

At the initial time of the trials, it was unknown whether or not a well-formed, uniform thickness, consolidated sheet could be produced with the proper physical properties due to the high coarseness, length distribution, and stiffness of the fibers. It was discovered that mechanical refining with typical refiner plate designs resulted in a paper design comprised of a distribution of fiber lengths containing a significantly higher percentage of fines and fibers between 0.2 and 0.5 mm and at the same time a higher percentage of fibers greater than 3.5 mm, which when used to produce a sheet yielded a paper that was well formed, of uniform caliper, consolidated, high bulk, highly textured “micro-roughness” with good internal bond and surface strength. Thus, it was confirmed that pulp containing coarse fibers might be refined and formed into sheets for glass interleaver paper.

It was found that the approach disclosed herein to making glass interleaver paper results in a paper with a superior ability to protect the glass from scratching and abrasions. Forming a sheet via this process and fiber length composition is a new and superior approach compared to, for example, making a sheet with fine northern fibers (as has been conventionally done) and was unanticipated when glass interleaver was first introduced to market.

In Table 2, below, various comparative data is provided detailing various paper compositions and trial results. In Table 2, the following formulations are compared: (1) a benchmark formulation of a competitive paper A; (2) NBSK [Northern bleached softwood kraft pulp], high refined; (3) NBSK [Northern bleached softwood kraft pulp], moderate refining; (4) NBSK, moderate refining, less uniform; (5) Coarse pulp, moderate refining; (6) Coarse pulp, high refining; (7) Coarse pulp (90%) and mercerized pulp (10%), moderate refining; and (8) Coarse pulp (90%) and mercerized pulp (10%), high refining. It can be noted that the condition produced with Coarse Fiber and moderate refining produced a slip sheet for glass interleaver which is a preferred embodiment of the invention with a superior combination of Density, Parker Smoothness, Surface Roughness, Void Volume, and Scott Bond versus currently useful approaches.

TABLE 2 Fiber morphology and sheet property data for trial conditions and benchmark (7) (8) Coarse Pulp Coarse Pulp (4) (5) (6) and and (1) (2) (3) NBSK, moderate Coarse Pulp, Coarse Pulp, Mercerized Mercerized Example Competitive NBSK, high NBSK, moderate refining, moderate high Pulp, moderate Pulp, high Description Paper A refining refining less uniform refining refining refining refining % Softwood Pulp 94.8 100.0   100.0 100.0 100.0 100.0 90.0 90.0 % Softwood 0.0 0.0  0.0 0.0 0.0 0.0 10.0 10.0 Mercerized % Hardwood 5.2 0.0  0.0 0.0 0.0 0.0 0.0 0.0 Length, length 1.596  1.836 1.9 1.932 2.069 1.645 1.882 1.543 weighted mean (mm) Width (um) 27.1 26.2  25.9 26.5 28.9 28.2 29 28.3 Coarseness (mg/m) 0.160  0.1424 0.144 0.142 0.224 0.236 0.239 0.235 % Fines 4.12 4.25 3.31 3.19 8.94 6.64 3.94 5.05 % fiber length < 0.5 10.8 8.4  6.8 6.8 9.6 17.1 13.4 19.2 mm % fiber length > 3.5 2.8 4.1  4.7 5.4 17.8 8.4 11.7 6.2 mm % long fiber plus short 13.6 12.5  11.5 12.2 27.4 25.5 25.1 25.4 fiber % medium length fiber 61.0 60.9  63.4 60.8 47.2 47.7 45.8 47.0 0.9 mm to 2.7 mm Basis Weight 31.0 30.9  30.9 31.0 30.7 30.7 30.7 30.7 (lb/3000 ft2) Caliper (mils) 3.33 2.95 3.11 3.20 3.54 3.33 3.37 3.46 Density (g/cm3) 0.60 0.67 0.64 0.62 0.56 0.59 0.58 0.57 Porosity (sec/100 ml) 12 117    33 28 7 128 35 106 Parker Smoothness 8.74 8.67 8.66 8.47 11.35 10.87 9.51 11.44 Felt (um) Parker Smoothness 7.40 7.30 6.87 6.92 9.41 9.54 11.30 9.94 Wire (um) Scott Bond (g) 118 347+    237 234 300 344 306 310 Void Volume (g/g) 1.63 0.98 1.34 1.25 1.63 1.35 1.35 1.16 Surface Roughness Sa 5.22 4.36 4.56 4.98 6.85 5.23 6.60 5.34 Felt (um) Surface Roughness Sa 4.34 3.66 3.76 3.74 5.44 5.20 4.87 5.13 Wire (um) % Ash 0.23 0.10 0.10 0.10 0.28 0.10 0.10 0.10

Fiber analysis confirmed that the unprocessed fibers and refined fibers from the paper had high levels of fiber coarseness, defined as the weight of fiber per length. Fiber coarseness is a result of the thickness of the fiber wall and the overall fiber diameter. Fibers with thick walls and narrow lumens (hollow area in center) have high coarseness while fibers with thin walls and wide lumens have low coarseness. Mercerized pulps are produced with a chemical process that creates “kinks” in the fiber, which creates a high coarseness measurement. It was also surprisingly learned that the coarse pulps when refined using the same conditions as the northern fibers resulted in a similar average fiber length after refining than the less coarse, northern fiber pulps. This difference is a result of increased cutting in the refining process, rather than effects that just cause fibrillation and collapse of the fibers. The refining equipment and intensity produced a broad fiber distribution of fiber fractions with a high percentage of low fiber length fractions compared to northern fiber sheets. There is also a surprisingly large percentage of long fibers remaining. This broad distribution of fiber lengths containing a higher percentage of long and short fiber fractions results in a well formed sheet with high bulk, anti-linting, and roughness properties that provide superior glass protection.

This enables the sheet to contact the glass at more points which lowers the specific pressure against the glass surface and creates surface voids that debris can go into/be trapped by which prevents contacting of the glass surface and scratching of the surface. The sheet also retains its bulk because of the coarse fibers, so the sheet is a better cushion for the glass. The refining also increases the bonding area and enables the fibers to bond to one another so they do not transfer to the surface of the glass. This new combination of attributes lowers the sheet's propensity to scratch or abrade the glass surface, while resistance to linting or other material transfer versus the standard northern fiber based sheets. Northern fiber sheets treated using this technique will not maintain the same bulk, provide large enough surface voids, and cannot cushion the glass as well.

Accordingly, an improved paper for slip sheet interleavers for contacting glass surfaces for separating glass sheets in which the paper is produced using fibers with high coarseness. The refining level applied could be as high as 18 hp-day/ton but would likely be more specifically in the range of 7-16 hp-day/ton, and the sheet would retain bulk properties due to the coarseness of the pulps. The pulp length distribution in the superior performing sheet may contain greater than 5% of fibers in the 0.2 to 0.5 mm range, greater than 5% fines (i.e., less than 0.5 mm), and greater than 5% of the fibers greater than 3.5 mm. The pulp used could be produced without the use of a PDMS oil containing defoamer. The sheet may preferably be produced with one pulp source but multiple pulp sources could be used to achieve the fiber length distribution and sheet properties. Unlike many existing interleaver sheets, the improved sheet would not be calendered in order to retain high bulk properties.

The bulky, soft, high number of contact point sheet properties result in reduced scratching of glass due to contaminants from the paper or the glassmaking process [especially when packing unfinished LCD glass having contaminants from the bottom of draw (BOD) of the glassmaking process]. This improvement is especially valuable in view of the increasing demands for interleaver paper that reduces the number and size of scratching and abrasions packaging for display glass identified above for displays of increased resolution where there is increasingly less tolerance for damage to the glass.

The increased bulk and micro roughness in the newly disclosed interleaver formulations translate to a reduction in scratches on glass packed with the new paper. High levels of internal bond and surface strength will result in reduced particle generation. Reduction in scratching and particle generation reduces pixel loss when producing LCD/OLED device screens, improves glass sheet yield, reducing overall costs of the glass, and improving customer satisfaction from glass manufacturers as well as LCD/OLED panel manufacturers. The combination of surface texture, Parker smoothness, bulk, volume void, Scott Bond, fiber coarseness, and fiber length distribution, all contributing to a new level of scratch and abrasion resistance that is heretofore unseen.

In order to illustrate the effect of Coarse Fiber on paper surface topography and the corresponding improvement in the ability to protect glass surfaces from scratches and contamination, surface roughness profiles are shown in FIGS. 1 and 2. The charts in FIGS. 1 and 2 show the roughness of the paper surface, with the height of peaks and depth of valleys shown on the Y-axis in microns for a 1.2 mm span, shown along the X-axis in the cross direction of the paper. Simulated contaminants were included on the sheet surface profile and are shown as circles approximately 5 microns in diameter. Also in each figure, a simulated glass plate is shown against the paper surface to illustrate the number of touch points with the paper as well as contact points with the contaminants.

FIG. 1 shows the surface of glass interleaver paper produced with standard NBSK pulp. This figure clearly shows the high rate of occurrence of contact with a contaminant. This high number of contaminant contact points increases the number of defects on the glass surface, including scratches and stains.

In contrast, FIG. 2 shows the surface of glass interleaver paper produced with standard Coarse Pulp. This figure clearly shows the reduced number of contact points with a contaminant as compared to the NBSK paper. This lower number of contaminant contact points reduces the number of potential defects on the glass, including scratches and stains.

The present invention has been described in terms of one or more preferred embodiments, and it should be appreciated that many equivalents, alternatives, variations, and modifications, aside from those expressly stated, are possible and within the scope of the invention. 

What is claimed is:
 1. A paper used as a slip sheet interleaver for the protection of glass surfaces, the paper comprising a sheet with a basis weight of 15-60 lb/3,000 ft², having a bulk density less than 0.75 g/cm³, and an average fiber coarseness greater than 0.18 mg/m.
 2. The paper of claim 1, wherein the sheet comprises at least 10% coarse fibers.
 3. The paper of claim 2, wherein the at least 10% coarse fibers comprise a set of fibers selected from a group consisting of one or more of southern softwood kraft, southern hardwood kraft, mercerized fibers, Coastal Douglas Fir, Radiata Pine, Fluff Pulp, and synthetic polymeric fibers.
 4. The paper of claim 1, wherein the sheet comprises at least 80% coarse fibers.
 5. The paper of claim 4, wherein the at least 80% coarse fibers comprise a set of fibers selected from a group consisting of one or more of southern softwood kraft, southern hardwood kraft, mercerized fibers, Coastal Douglas Fir, Radiata Pine, Fluff Pulp, and synthetic polymeric fibers.
 6. The paper of claim 1, wherein the sheet has a basis weight of 25-40 lb/3000 ft², a bulk density less than 0.65 g/cm³, and an average fiber coarseness greater than 0.20 mg/m.
 7. The paper of claim 1, wherein the sheet has a Scott Bond strength greater than 200 g.
 8. The paper of claim 1, wherein the sheet has a Scott Bond strength greater than 250 g.
 9. The paper of claim 1, wherein the sheet has a Parker smoothness greater than 7.5 μm on both sides
 10. The paper of claim 1, wherein the sheet has a Parker smoothness greater than 8.5 μm on both sides.
 11. The paper of claim 1, wherein the sheet has a surface roughness (Sa) greater than 4.5 μm on both sides.
 12. The paper of claim 1, wherein the sheet has a surface roughness (Sa) greater than 5.0 μm on both sides.
 13. The paper of claim 1, wherein the sheet has a void volume greater than 1.30 g/g.
 14. The paper of claim 1, wherein the sheet has a void volume greater than 1.40 g/g.
 15. The paper of claim 1, wherein the sheet has an average fiber length (LWFLA) less than 1.9 mm.
 16. The paper of claim 1, wherein the sheet has an average fiber length (LWFLA) less than 2.1 mm.
 17. The paper of claim 1, wherein the sheet has a short fiber and fines content greater than 10% in which the short fibers and fines are less than 0.5 mm.
 18. The paper of claim 17, wherein the sheet has a long fibers content greater than 5% in which the long fibers are greater than 3.5 mm.
 19. The paper of claim 1, wherein the sheet has a content of medium length fibers between 0.9 mm and 2.7 mm of less than 55%.
 20. The paper of claim 1, wherein the sheet has a short fiber and fines content greater than 15% in which the short fibers and fines are less than 0.5 mm.
 21. The paper of claim 1, wherein the sheet has less than 0.5 parts per million (ppm) poly dimethyl siloxane (PDMS) content.
 22. The paper of claim 1, wherein the sheet has less than 0.35% ash content.
 23. The paper of claim 1, wherein the paper comprises a binding agent.
 24. The paper of claim 23, wherein the binding agent is selected from the group of adhesives consisting of acrylic latex, styrene butadiene copolymer, butadiene acrylonitrile copolymer, polyurethane, polyvinyl acetate, polyvinyl alcohol, natural rubber or other nature-based adhesive, polyvinyl chloride, polychloroprene, epoxy, phenol, urea-formaldehyde, and thermal melt adhesive.
 25. The paper of claim 23, wherein the binding agent is fiber comprised of at least one polymer selected from the group of polymers consisting of polyolefin, polyester, polyamide, polylactide, polycaprolactone, polycarbonate, polyurethane, polyvinyl acetate, polyvinyl chloride, polyvinyl alcohol, polyacrylate or polyacrylonitrile, and ionomer.
 26. A method of making the paper of claim 1, the method comprising the use of refining equipment, refining energy level, and fiber types to produce the paper comprising fibers having a wide fiber length distribution wherein the sum of short fibers and fines and long fibers is greater than 20% of the fiber mix in which the short fibers and fines are less than 0.5 mm and in which the long fibers are greater than 3.5 mm.
 27. A method of making the paper of claim 1, the method comprising the step of using of a paper machine forming section to make the paper, in which the forming section includes at least one of a Fourdrinier, Inclined Wire Former, Cylinder Former, Twin Wire Former, Gap Former, Top Former, Multi-layer Former, and Tanmo.
 28. A method of separating sheets of glass using the paper of claim 1, the method comprising separating two sheets of glass by positioning the paper therebetween. 